Leetcode: Subtree of Another Tree

Given two non-empty binary trees s and t, check whether tree t has exactly the same structure and node values with a subtree of s. A subtree of s is a tree consists of a node in s and all of this node's descendants. The tree s could also be considered as a subtree of itself.

Example 1:
Given tree s:

     3
    / \
   4   5
  / \
 1   2
Given tree t:
   4 
  / \
 1   2
Return true, because t has the same structure and node values with a subtree of s.
Example 2:
Given tree s:

     3
    / \
   4   5
  / \
 1   2
    /
   0
Given tree t:
   4
  / \
 1   2
Return false.

 1 class Solution {
 2     public boolean isSubtree(TreeNode s, TreeNode t) {
 3         if (t == null) return true;
 4         if (s == null) return false;       
 5         if (isSametree(s, t)) return true;
 6         return isSubtree(s.left, t) || isSubtree(s.right, t);
 7     }
 8     
 9     public boolean isSametree(TreeNode s, TreeNode t) {
10         if (s == null && t == null) return true;
11         if (s == null || t == null) return false;
12         if (s.val != t.val) return false;
13         return isSametree(s.left, t.left) && isSametree(s.right, t.right);
14     }
15 }